Why did evolution turn crabs into crabs and crabs into crabs? – News Home



米。 1. 十足类动物的简化系统发育树

Over the course of evolution, decapods became crabs many times—they acquired their characteristic flat, wide bodies and short abdomens. Even more surprising, they often lost their “crabs”, again approaching the shape of common crayfish. Scientists have even come up with specific terms for these changes and are trying to understand why with the help of modern technology. Several hypotheses have been proposed, but none of them can be considered satisfactory so far due to data gaps.

Traditionally, “crab” refers to the suborder braculawas included in the lineup decapods (full head).In taxonomic terms, they are often opposed to suborders Nomuraor “fake crabs”, differ from them in the number of walking legs.

But the two suborders have more in common than differences. Their anatomy is very similar – they have a broad cephalothorax covered with a flat shield – carapace. abdomen (pleon), usually long in crustaceans and looking like a thick cancerous “tail”, in crabs – “true” and “false” – reduced and curved under the cephalothorax. The external morphological features of crabs are accompanied by internal ones – some abdominal ganglia fuse into one, and abdominal muscles are reduced. It is reasonable to assume that two parallel (convergent) suborders acquired such a structure during evolution.

It turns out to be more interesting – paleontological and phylogenetic studies have shown that within each suborder, crab body-planning traits developed independently several times: twice for “true” crabs, and twice for false crabs. Evolutionary standards repeat this process so often that a special term has been coined for it: cancerization (carcinogenic)。

And, within the same taxon—both short-tailed and Nomura—the “crab” disappeared several times: the carapace became long and narrow again, and the belly long and straight. There is also a separate term for this process – decarbonization. It seems natural selection has been experimenting with the decapod crayfish as to how best to make the crab – and if he “didn’t like it”, he would redo everything.


米。 2. 螃蟹的简化系统发育树

In general, the convergent development of the same trait within a taxon, and its loss, is a fairly common occurrence in evolutionary biology, even in epoch-making events like this. multicellularity. For example, in Volvo In green algae – that is, in a fairly small taxon (about 60 species in 15 genera) – multicellularity successfully occurs twice, while cellular differentiation – up to four times! What can we say about canceration involving much smaller changes in the body’s structural plan?

First, the “age” in question sparked interest in carcinogenesis (and de-cancer): the term itself was introduced as early as 1916. Second, crabs are more “tangible” and culturally meaningful to a broad audience than multicellular organisms.

In general, the evolutionary theme of crabs is quite “evergreen”, so scientists continue to study it, especially looking for answers to the question: why does evolution come to the “crab” structure so often and “reject it so easily” “?

recent review About this topic was published in the magazine last year biology paper. The journal’s name speaks for itself: it publishes articles on controversial issues in biology for which there is no clear solution. Scientists are considering several possible explanations.

The first explanation that comes to mind is some general selection factor that causes carcinogenesis in decapods. But it’s not clear which factor might play this role. Crabs (“true” and “false”) inhabit a wide range of habitats, from mountain rivers to the deep ocean, from coral reefs to enclosed caves. It is difficult to single out some general selection factors that force the transition from oncogenic to de-cancerous here. Clearly, the wide carapace facilitates movement through a lower center of gravity and provides better protection from predators. But so far, the available fossil and molecular phylogenetic data do not allow us to detect any correlation between carcinogenesis/decarcinogenesis and the presence of predators and the environment in which people must move. Another hypothesis remains in the same state of elusiveness: Cancerization makes the jaws of crayfish more flexible, making them easier to hunt.

Finally, there is a completely opposite hypothesis: the tendency toward cancer and de-cancer is not caused by external factors, but by the peculiarities of the decapod anatomy itself. The changes necessary for cancerization (and decancerization) are closely related anatomically: for example, abdominal curvature requires fusion of abdominal ganglia and reduction of abdominal muscles. Whenever a decapod crayfish appears somewhere with a slight curvature of its abdomen, it starts the process of turning into a crab. However, the characteristics of decapods’ internal structure are as varied as their habitats — so this assumption is also problematic.

To draw reliable conclusions from such a diverse group of animals, more genomes and fossils are needed where biostatistics and bioinformatics methods can reveal any significant correlations. But this data is still full of gaps. “Almost half the branches of the crab evolutionary tree are shrouded in darkness”,—— Write author of the article. A simpler conclusion sounds like this: For over a hundred years, the world has been puzzled by crab evolution because there are many crabs but little data.

resource: Joanna M. Wolfe、Javier Luque、Heather D. Bracken-Grissom。 How to Be a Crab: Phenotypic Limitations of Repeated Body Programs // biology paper. 2021. DOI:10.1002/bies.202100020。

George Kuragin



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